1. Field of the Invention
The present invention relates to method and apparatus for encoding and decoding color image data for efficiently compressing and storing the color image data.
2. Description of the Prior Art
It is known in the art to compress the color image data by converting image data comprising red (R), green (G) and blue (B) signals, expressed in RGB coding system, to image data comprising a luminance signal (Y) and color difference (chrominance) signal (I, Q), expressed in a YIQ coding system. In the RGB coding system, there is a typically a large amount of redundancy due to frequent correlations between the colors. However, in the YIQ coding system, such a redundancy is eliminated because the luminance and chrominance signals are expressed with orthogonal functions. Thus, by the conversion of image data from an RGB system to a YIQ system, the amount of image data can be reduced greatly. Furthermore, under the YIQ system, since the human eyes are less sensitive to chrominance than luminance, the chrominance signal (I, Q) can be coded with rough quantization, resulting in the reduction of the total data amount.
In FIG. 1, a prior art converter employing a prior art encoding and decoding system is shown for converting image data from an RGB system to a YIQ system, thereby compressing color image data to be stored, for example, in a floppy disk. A full color still picture image, which is expressed by a digital signal in the RGB system (having RGB data including R data, G data and B data) is applied to image input device 11. The RGB data is converted to digital signals in the YIQ system (having YIQ data including Y data, I data and Q data) by YIQ converter 12. The conversion in YIQ converter 12 is carried out based on the following equation (1): ##EQU1##
The converted YIQ data is applied to encoder 13 in which the Y data, I data and Q data are quantized and encoded to Y1 data, I1 data and Q1 data. The encoded data are recorded (e.g., on a floppy disk) in a recording/play device 14. According to this prior art encoding system, Y, I and Q data are encoded linearly to Y1, I1 and Q1 data, as shown in the graphs depicted in FIGS. 2a, 2b and 2c, in which the abscissas and ordinates, respectively, represent input and output data of encoder 13. FIG. 2a shows that each eight bit word of luminance data (Y), which express e.g., one out of 256 levels (0 through 255), is linearly encoded to an eight bit words of luminance data (Y1) which can express the same 256 levels. FIG. 2b shows that each eight bit word of first chrominance data (I), which can express up to 256 levels (-128 through 127), is linearly encoded to a five bit word of first chrominance data (I1) which can express 32 levels. FIG. 2c shows that each eight bit word of second chrominance data (Q), which can express up to 256 levels (-128 through 127) is linearly encoded to a four bit word of second chrominance data (Q1) which can express 16 levels. Since the human eyes are less sensitive to variations in chrominance than in luminance, the first and second chrominance signals (I, Q) are coded with rough quantization.
Then, in order to display the image on the display device 17, the recorded Y1, I1 and Q1 data are read out and linearly decoded in decoder 15 back to Y, I and Q data. The decoding in decoder 15 is effected in the reverse relationship, in terms of input and output, of the graphs shown in FIGS. 2a to 2c. Thereafter, the Y, I and Q data are converted to R, G and B data in RGB converter 16. The conversion in RGB converter 16 is carried out based on the following equation (2). ##EQU2##
The above described color image data encoding and decoding method does not taken into consideration the visual recognition characteristics of human eyes. Thus, the compressed data which is stored, e.g., on floppy disk, is not properly compressed.